One more alternator question (F-L 120)...

Still trying to figure out what killed my alternator.

When I started investigating the problem I found that the drive belt was quite loose (as in, I could turn the alt easily by hand without the belt moving). This was totally my bad, as the alt was a new install about a year ago and I know that drive belts stretch so I should have checked it a couple of times. But there was other bigger stuff going on with the engine last spring (head gasket job) that distracted me.

So here's the question: can a loose belt (with increasing slippage over time) cause alternator burnout?

I really am hoping to find some simple and immediate cause that doesn't involve a long, frustrating search for some subtle flaw in the larger electrical system

I will have to replace it no matter what, so... is there a preferred brand of alternators for Ford Lehman 120's?

40 years in and around auto service business. Never heard of a loose belt ruining an alternator. I would tighten the belt and test the charging system / battery as a whole before replacing anything. Maybe something else loose? Connections?

Cigatoo said:

40 years in and around auto service business. Never heard of a loose belt ruining an alternator. I would tighten the belt and test the charging system / battery as a whole before replacing anything. Maybe something else loose? Connections?

Click to expand...

thanks, that's not the answer I would have preferred but it's a good solid answer. eliminating one thing after another, eventually I may figure out what's the real problem!

I have HEARD of loose belts heating an alternator to the point the alt. fails.
Long time ago and no experience with it pesonally but maybe ask at an alternator rebuild shop.

Odd though that the belt and alt. also would not be hot and stinking of burned rubber and wiring. But ask.

My understanding is that if an alternator doesn't have anything to charge - it will burn out. How does that happen? - (master) battery switch turned off or in our case, on our GB - the shunt that feeds the amp meter at the helm was burned out (like melted).

How do you know the alternator is dead?

You may be able to get an alternator shop to rebuild it for pretty small $$.

Alternatively you could source a replacement - but also consider upgrading depending on how the rest of your systems are configured. We dropped in a nice externally regulated Balmar which wasn't cheap but provides a very valuable high current (100a+) charging source to augment solar on our non-generator boat.

Your question, unfortunately, begs other questions.

Is the current alternator an OEM and are you sure it is toast? I'd go to an alternator shop to have it checked out if you aren't.
Do you have twin engine setup?
An alternator is used to power the batteries. What are your future plans on using the boat? Planning on anchoring out?
Since you don't want to search out any issues, which you should do if the alternator is toast, then you are risking something bigger being in play. Not a wise choice.

Answer some questions and you might get better help.

BTW, you should know about American Diesel for any Ford Lehman issues. Look them up on line and use them, they are the ONLY Ford Lehman experts.

Good luck..

Depending on how an alternator is wired into the battery system, switching between batteries while the alternator is running can cause damage. This was discussed in your other thread, and you said you didn't do this. So that's not the cause.

The next killer of alternators is running them too hard for too long. Other than a select few large frame alternators, none are designed for continuous full output. And most don't protect themselves, but rather depend on the quick voltage rise in lead batteries to limit the time at full output. Worse yet, many assume they are only charging a start battery which will recover very quickly, limiting full output to minutes. Now hook that alternator to a house bank and it takes longer before output reduces. Make the house bank bigger and it takes even longer. There are no specs for how much and for how long an alternator can output full power, so it's all a giant guessing game. External regulators address this by sensing the alternator temperature, and limiting output as needed to keep the alternator from overheating. It essentially derates its continuous output rating.

It's a long way of saying that perhaps your alternator died from too much full power demand for too long, but there is no way really to tell.

I think all you can do it replace it and try again in hopes that it was just infant mortality. But if it fails again, then you still have a problem.

In the mean time, you might want to set up to monitor alternator output current, and alternator case temperature. Current can be checked with a clamp on meter, or you could install a shunt and fixed meter. I'd sugget checking temp with a contact probe, not an IR gun. I have found the IR guns to be wildly inaccurate depending on the surface material. For example, and IR gun reads 50-70F higher than actual on my alternators.

You are getting a lot of info and I think that it would be worth your time & money to get an ABYC Electriddian to come to the boat & investigate.

Before that YOU need to decide how you are going to use your boat.

I bought a boat in '10 to eventually do the Loop and in '23 we finish an 8yr Loop. I have a large batt bank with 2 large alternators (125A) 2 regulators (which control alternator output) 2 duo-chargers and a centerfielder (all Balmar equipment). We also added a solar bank while we were on the Loop. This is controlled by an MPPT.

Electrical systems that are designed properly have controls that keep the alternator from over charging your batt system. For example the centerfielder is set up to charge both the house bank and the engine batts. The regulators are there to take the alternator off-line so it doesn't fry your batts. An OEM alternator my not have a regulator or it may be built in. I think the batts would go before the alternator.

Without seeing your boat and how it is setup it's hard to provide adaquate info on your issue which is why an ABYC perwson would be best to ananlse what is going on. Remember, it's a boat and when you bought it you had to know that there were going to be expenses.

Welcome to the club.

Most auto parts stores well test your alternator for free.
You can rebuild it yourself. It's not hard.
Where I get parts:
There are others, Do a search.
There are Youtube videos of alternator rebuilds.

Thanks for the link!

Did you increase the output of the alternator? If so, the battery wire is probably undersized and getting hot which cooks the voltage regulator and diode trio.